Variations in the Earth's density result in small variations in the Earth's gravitational field. These variations can be measured at the Earth's surface using sensitive gravity meters. Through such measurements, masses of greater or lesser density than surrounding formations may be detected.
Gravity surveying is one technique in modern exploration for geophysically significant subsurface anomalies (or irregularities) potentially associated with ore bodies or hydrocarbon deposits, such as mineral and petroleum commodities. For example, the gravitational anomaly of a body of ore with a density contrast of 300 kg m−3 and a dimension of 200 m buried at a depth of 100 m is typically 2×10−6 ms−2, which is 0.00002% of the normal Earth gravity field. This relatively small effect is normally measured in units of milli gals (mGal), which is the unit for the free air and Bouguer gravity field measurements and is equivalent to 10−5 m/ss. Thus, for the above example, the body of ore would be represented by 20 mGal.
Currently, many gravity measurements have been made using instruments of the LaCoste/Romberg type that are essentially ultrasensitive spring balances detecting a small difference in weight caused by the gravity anomaly. The measurements are subject to a wide variety of environmental influences, and the measurements should be performed relative to a standard point that is used regularly during the survey as a fixed reference for removal of drifts in the instrument. This procedure can be slow, and may require extensive information on local topography and geology since a normal variation of gravity with height is approximately 0.3 mGal per meter, for example. Within moving platforms, such as aircraft, using this type of relative gravity instrument can be difficult for several reasons including the fact that the use of precision radar altimeters and pressure sensors to achieve vertical position to as little as one meter can impose limitations on the order of a few hundred mGals on the gravity data, for example.
For this reason, some geophysical prospecting has progressed towards gradiometry. In principle, measurement of a gradient of a gravity field over a known baseline allows accelerations due to motion of the platform itself to be cancelled out. Thus, higher precision gravity measurements can be recorded via gravity gradients. Gravity gradients are the spatial derivative of the gravity field, and have units of mGal over distance, such as mGal/m. The standard unit of gravity gradiometry is the Eötvös (E), which is equal to 0.1 mGal/kilometer or 10−9/s2 (e.g., gradient signatures of shallow Texas salt domes are typically 50 to 100 E).
Precision and accuracy of gravity gradiometer data depends upon the precision of the measurement device and the physical conditions at the point of observation. For example, nearby hills rising above the elevation of adjacent land cause a change in gravity readings. Likewise, nearby valleys cause a change in gravity readings due to a deficiency of attractive mass. However, the extent to which gravity gradiometer data does not match the terrain indicates detection of geophysical subsurface anomalies potentially associated with ore bodies or geology associated with hydrocarbon deposits. Therefore, gravity gradiometry data requires a “terrain” correction to account for these topographic effects that cause changes in gravity readings to expose possible geophysical subsurface anomalies.
In many gravity surveys, a largest component of the recorded gravity gradient signal originates from the terrain. Calculations to remove the terrain effect may require a knowledge of the local topography and density of surface rocks in the vicinity of each gravity reading. The portion of a recorded gravity gradient signal due to the terrain is usually removed by a manual application of a-priory geophysical knowledge. Density of the terrain near the surface is estimated based on a combination of available geological information and general geological concepts.
However, subsurface anomalies and terrain can have varying densities. Without accurate density values for the surface terrain, modeling of deep subsurface structures can be incorrect. Thus, when performing a gravity survey, a signal due to gravity gradients from subsurface anomalies and a signal due to the terrain will need to be analyzed and processed using different values of density. Thus, a means of processing gravity gradiometry signals using varying density values is desired.